Sunday, September 06, 2009

Crocodile myths #1 - the curious trochilus

I intend to make this a continuing series of articles, at least until I run out of crocodile myths to discuss. There are, as you might infer, no shortage of wild and wonderful myths associated with crocodiles (and alligators). Many of these seem to originate with the Greek philosopher Herodotus, and if he were still around I'd have a bone to pick with him. Whether he was misinterpreting crocodile natural history, or whether he was simply making stuff up is hard to say, but his influence has reached across the millennia.

One of the most popular myths is that of "trochilus" as Herodotus, and later Aristotle and Pliny, call it. This is supposed to be an Egyptian bird that flies into the mouth of a basking crocodile and feeds on scraps of food and leeches attached to the jawline and tongue. Herodotus describes it as follows: "...for the crocodile, when he leaves the water and comes out upon the land, is in the habit of lying with his mouth wide open, facing the western breeze: at such times the trochilus goes into his mouth and devours the leeches. This benefits the crocodile, who is pleased, and takes care not to hurt the trochilus." Many have attributed this behaviour to the Egyptian plover (Pluvianus aegyptius) which is a very common bird often seen around basking Nile crocodiles, although there's no proof that this is what Herodotus was referring to.

This description of a bird that cleans the teeth of crocodiles has undoubtedly entered popular culture, so much so that crocodiles are often used by advertising agencies to promote dental hygiene. But is it true? Do plovers or indeed any other birds actually clean the teeth of crocodiles?

I'll begin my answer with the opinion that I don't believe a word of it. However, there's more to this story than just a simple yes or no. First off, there's no evidence anywhere in photographs or film to show birds cleaning crocodile teeth or ripping leeches from their tongue (with the exception of that particularly clever digital fake you see above; click on it for the full version) and no published reports of it in peer-reviewed literature. I'd have thought a mutual relationship of this kind would have been easily observed by now. Secondly, contrary to popular belief crocodiles do not need their teeth cleaning. They regularly shed their teeth and replace them with new ones: each visible tooth has a solid tip but a hollow base and inside this base, like a set of formidable Russian dolls, are smaller teeth waiting to emerge. Tooth decay, broken teeth and staining are never a permanent problem for a crocodile. Thirdly, food simply cannot get stuck between their teeth - they are too widely spaced for food particles to get jammed in there, and they are regularly washed with water every time the crocodile slides off the bank. While bacteria and microscopic particles can indeed become prevalent around the base of the teeth, these are not problems that are going to be solved by the pecking of a bird large or small. Leeches are another matter, and crocodiles certainly suffer from these insidious passengers. It's generally thought that gaping the mouth during the day helps a crocodile to dry its mouth and hence discourage leeches, but do birds also help out? If they do, it hasn't been documented as such.

So what's going on? Am I just a born skeptic? Perhaps I am, but that doesn't mean there isn't something in this compelling relationship. Birds of various species are often found feeding in close proximity to crocodiles, and immobile crocodiles basking on the bank in the sun are rarely if ever concerned about birds wandering between them, standing on their back, or straying close to their jaws. Birds are opportunists too, and they will feed on flies and other insects on and around crocodiles. It may even appear that they are removing flies from the crocodile's jaws on occasion. But extrapolating this into a mutual relationship between crocodile and bird is going a bit far. Crocodiles hardly benefit from the presence of the birds, and yet they tolerate their presence because it's just not worth chasing them. Crocodiles aren't always in the mood for feeding, and they're smart enough to let difficult to catch prey like alert birds pass them by.

So if you ask me, the crocodile bird exists in name only. There is no mutual relationship between them, as none has ever been seriously documented, no advantage would be gained by the crocodile, and the hypothesis just doesn't add up. Of course, I like to think that I'll reconsider anything based on actual evidence. I wait in hope.

8 comments:

This is a hot topic in ornithology as well as at almost every campfire in Africa. The bird in question is said to be either Egyptian plover or wintering white wagtail. Supposedly there is a documentary somewhere showing this behavior, but I haven't been able to locate it.

Personally, I find it totally believable that a bird could start picking insects from the gums of a basking crocodile. But regular symbiotic relationship... no way.

Lauri, far be it for me to diminish art, and I am sure that poetry can be written about this still. Perhaps it can serve as an allegory to the power of myth rather than something to be taken literally?

Wow very funny commercial and you can tell it is CG when you see the whole thing, bummer! I'll keep looking as it has been a childhood belief as I could have sworn I saw some Mutual of Omaha show, or its equivalent, on it. Thanks for your reply. John

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I'm a zoologist specialising in crocodiles, whether it's research, management, conservation or film-making. I run the popular website crocodilian.com and a consultancy business called Big Gecko with my wife Erin.